1. Field of the Invention
This invention relates to the condensation of isocyanates with carboxylic acids or anhydrides to form imides, amides, or amide-imides and is more particularly concerned with the use of cyclic phosphorus compounds as catalysts for the condensation.
2. Description of the Prior Art
The reaction of isocyanates with carboxylic acids or anhydrides to form imides and amides, and, more particularly, the reaction of organic diisocyanates with dicarboxylic acids or dianhydrides, or anhydrides containing free carboxyl groups, to form polyamides, polyimides or polyamide-imides, is well-known; see, for example, U.S. Pat. Nos. 3,592,789; 3,541,038; and 3,708,458. The use of catalysts to promote these condensations is also known; see, for example, U.S. Pat. No. 3,701,756.
We have now found that the above reactions can be catalyzed in highly satisfactory manner using a group of heterocyclic phosphorus compounds which have not hitherto been suggested for this purpose. Our findings are particularly surprising in view of the known behaviour of this class of phosphorus compounds as catalysts for the conversion of isocyanates to carbodiimides. We have found that the compounds in question can be used to catalyze the above reactions of isocyanates with carboxylic acids and derivatives thereof without any significant concurrent formation of carbodiimides except in certain instances.